Generally, relating to the phase separation due to coacervation, there have been well known salt coacervation, simple coacervation and complex coacervation effecting phase separation into two phases of thick phase and weak phase obtained by mixing colloidal aqueous solutions more than two kinds having opposite electric charges by adjustment of pH, and this coacervate of thick colloidal phase can be utilized as wall film of microcapsules. That is to say, if at the time of production of coacervate, water insoluble material such as oil droplets and solid powder previously coexisted in this system, coacervate is produced using these particles as cores, and thereby microcapsules can be obtained. Such a capsule producing technique has a wide area of use according to materials packed in the capsules, and therefore studies have been made about production of non-carbon paper as well as applications to recording material, medical supplies, spices, agricultural medicines, chemical synthetic products, adhesives, liquid crystal paint, foodstuffs, cleaning material, dyestuffs, solvent, catalyst, enzyme, rust proofing material and so forth, and these have practically been used.
Heretofore, as polycation of hydrophilic colloidal material having ionized group of capsule raw material in complex coacervation of this aqueous solution system, acid treated gelatin, i.e. gelatin having an isoelectric point of 8 to 9 plays a central part, and as a polyanion capable of forming gelatin and coacervate, there are gum arabic, sodium alginate, agar, carboxymethylcellulose (CMC), and also copolymers of synthetic organic compounds, such as poly (styrene-maleic anhydride), poly (ethylene-maleic anhydride) and poly (vinylmethylether-maleic anhydride) and so forth can be used. However these copolymers are remarkably different from gum arabic etc. in their characteristics, and in general have too large a hydrophobic property and are easy to aggregate together, and accordingly when contacting with gelatine it is easy to produce an insoluble massive precipitate rather than coacervate, and therefore they are suitable to form large-sized tough capsules but unsuitable to microcapsules.
In addition, there has been known a method of causing the phase separation from the mixed aqueous solution of gelatin and polyvinyl alcohol, however this is not practical, and at present a method of producing microcapsules by complex coacervation of the combination of acid treated gelatin and gum arabic is considered best and has been utilized most widely.
However, in order to obtain practical fine capsules by producing coacervate from a mixed aqueous solution of this gelatin-gum arabic, there are some problems such as:
1. It is necessary to use gelatin and gum arabic in the same concentration and amount; (2) A large capsule producing arrangement is required because of the necessity for a of dilute solution of a concentration of aqueous solution lower than 3%, (3) Amount of hydrophobic material usually used is optimum when it is double parts by weight of the wall film colloidal material, and a large amount of oil cannot be capsuled effectively; (4) Operations of capsule forming steps are complicated; (5) The wall film is molecularly porous, and has such a permeability that the content (i.e. oil) of low molecular liquid material permeates therethrough at a higher temperature; (6) It is required to use acid treated gelatin having a pH of isoelectric point of 8 to 9.
Moreover, since the capsules are obtained in the dispersed dilute condition, said dispersion liquid must be further concentrated at the time of practical application of such capsules.